Layered heaters are typically used in applications where space is limited, where heat output needs vary across a surface, where rapid thermal response is desired, or in ultra-clean applications where moisture or other contaminants can migrate into conventional heaters. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the electrically-live resistive material and also reduces current leakage to ground during operation. The resistive material is disposed on the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to an electrical power source, which is typically cycled by a temperature controller. The lead-to-resistive circuit interface is also typically protected both mechanically and electrically from extraneous contact by providing strain relief and electrical isolation through a protective layer. Accordingly, layered heaters can be highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among other types, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film dispensing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another series of processes distinct from thin and thick film techniques are those known as thermal spraying processes, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
In hot runner nozzle applications for injection molding equipment, a variety of heaters have been used and are typically disposed around the outer diameter of the hot runner nozzle body. Such heaters have often proven difficult to remove for repair or replacement due to thermal expansion between the hot runner nozzle body and the external heater during operation. Additionally, many hot runner nozzle heaters have been time consuming and costly to produce, in addition to demonstrating certain inefficiencies in providing the requisite temperature profiles along the length of the hot runner nozzle throughout a variety of different processing environments.